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No such thing as a constant constant?

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RICHARD FEYNMAN called the fine structure constant, alpha, “one of the greatest damn mysteries of physics&colon; a magic number”. One puzzle is whether this constant of nature has always had the same value. A signal from the early universe could answer that.

Alpha determines the strength of the electromagnetic force. Its value can be calculated by studying the so-called 21-cm line in the spectrum of neutral hydrogen atoms in the universe. The spectrum has a blip at a wavelength of 21 centimetres because the atoms absorb light at this wavelength in a manner that depends on alpha.

To find out if alpha has always been the same, Ben Wandelt and Rishi Khatri at the University of Illinois at Urbana-Champaign suggest measuring the 21-cm line from about 400,000 years after the big bang, when neutral hydrogen formed, to about 150 million years later when the first stars flared up.

As you go back in time the intensity of the 21-cm spectrum should follow an expected pattern that mirrors the varying amounts of neutral hydrogen at different times. Deviations from this would mean alpha had other values in the past.

So far, physicists have only studied alpha using the cosmic microwave background – the radiation left over from about 400,000 years after the big bang – but the 21-cm radiation can see variations at different times. The information we have about alpha “could be up to 10 times better”, says Wandelt.